Use mypy to check all Python demo programs.
Updated the demos to pass type checking.
There were a couple of small mistakes found, so this was worth the effort.
The demo program ecliptic_of_date.py shows how to
calculate the true ecliptic of date (ECT) angular coordinates
of the Sun, Moon, and planets for an observer somewhere on the Earth.
It calculates the equatorial of date (EQD) coordinates, then uses
a rotation matrix to convert the vector to ECT, then converts
the vector to spherical coordinates: latitude, longitude, and distance.
I made the scripts for testing the demos for
C, C#, JavaScript, and Python follow the improved
pattern used for Java and Kotlin: much smaller
and easier to maintain thanks to bash functions.
I refactored the unit tests for all the demo programs
to follow a different pattern that makes it simpler
to add more demo tests in the future.
The main thing is that correct output and generated
output are now in separate directories `correct` and `test`.
I have moved the test scripts from `test/test` to `./demotest`
in all the langauge demo directories.
This makes it simpler to clean up any stale generated
files before each test run by `rm -f test/*.txt`.
I stumbled across this while making the Java demo tests,
and it was a better solution, so now all the other languages
are consistent with the Java demo tests.
In the C demo tests, I also decided to compile all the
binary executables into a subdirectory `bin` that can
be cleaned out before each run, to make sure there are
no stale executables from an earlier run.
